1. Field of the Invention
The present invention is directed to a direct-current ultra-high-pressure mercury lamp used as a light source for liquid crystal projector equipment and DLP projector equipment.
2. Description of the Related Art
Light source equipment with a short-arc discharge lamp within a convex reflector mirror made of borosilicate glass is conventionally used for liquid crystal projector equipment and DLP projector equipment. However, because projector equipment is required to project an image evenly and with adequate chromaticity, metal halide lamps that incorporate mercury and metal halides and that have good chromaticity have been used as light source lamps. Smaller and lighter equipment is also highly desirable, and thus, discharge lamps must be made smaller.
Recently, there has been movement toward smaller lamps as point light sources, and there has also been an increase demand for discharge lamps with very short interelectrode distances. However, in metal halide lamps that incorporate a metal with a lower energy of excitation than mercury, if the interelectrode distance is less than a certain amount, there are limits to how much the brightest spot can be concentrated. Accordingly, it becomes difficult to make a smaller point light source. For that reason, short-arc ultra-high-pressure mercury lamps having a very high mercury vapor pressure when lit, for example 20 MPa or higher, have come into use in place of metal halide lamps. In order to have such a high mercury vapor pressure value when lit, at least 0.15 mg/mm3 of mercury is incorporated within the tube. In such an ultra-high-pressure mercury lamp, arc spread is controlled, and it is possible to enhance further the light output and improve chromaticity. Such ultra-high-pressure mercury lamps have been presented in U.S. Pat. No. 5,109,181 and U.S. Pat. No. 5,497,049.
In view of the aforementioned related art problems, there is a requirement for lamps with greater light output, superior chromaticity, and also longer service life. Specifically, it is desirable that, while the lamp is used in projector equipment, the radiant intensity of light from an ultra-high-pressure mercury lamp does not drop or change, but be maintained as stable as possible. As stated above, however, small size is required for a ultra-high-pressure mercury lamp used as a light source for projector equipment, thus, small tubes with volumes not exceeding 300 mm3 are used. Consequently, the load on the tube wall is great, and the temperature within the tube reaches 950xc2x0 C. to 1050xc2x0 C. For that reason, over long hours of use, the quartz glass that is the material of the tube gradually opacities and loses transparency, and because the transparency to light declines, the radiant intensity of the light drops. Moreover, the temperature of the electrode tip reaches 2500xc2x0 K or above; because the temperature is very high, impurities included in the tip of the electrode, which is made of tungsten, evaporate and wastage occurs. Then the vaporized materials adhere to the inner wall of the tube, and darken the tube. This reduces the transparency to light and causes deterioration of the light output, and also reduces the transparency of the quartz glass. In particular, the cathode is smaller than the anode and has a smaller heat capacity, and its sharply pointed tip is liable to wastage.
Accordingly, it is an object of the invention to provide a direct-current ultra-high-pressure mercury lamp which has a tube that does not lose transparency even during long hours of use, exhibits low waste of the electrodes and particularly the tip of the cathode, and a longer service life than conventional lamps.
These and other objects are achieved by a direct-current, ultra-high-pressure mercury lamp in which a cathode and an anode of tungsten face each other within a quartz glass tube. The cathode is preferably composed of tungsten doped with potassium and includes a cathode coil wrapped around the cathode rod and which is composed of tungsten having a purity of at least 99.99%. The anode is preferably composed of tungsten having a purity of at least 99.99%.
In conventional ultra-high-pressure mercury lamps, the electrodes have been formed of tungsten with a purity of at least 99.9%. Tungsten with a purity of at least 99.9% has metallic impurities including about 60 wt-ppm (hereafter xe2x80x9cppmxe2x80x9d) of K, as well as many others such as Fe, Al, Si, Mo, Ni, Mg, Cu, Mn and Na, for a total of 100 ppm to 1,000 ppm. The lamp is very hot when lit and, as stated previously, these metallic impurities vaporize and adhere to the inner wall of the tube. Not only does the tube darken because of them, but the inventors discovered that when the temperature is raised to 1000xc2x0 C., quartz glass crystallizes with these adhered impurities as nuclei, hastening the loss of transparency. Accordingly, in accordance with the present invention the anode is formed of tungsten with a purity of at least 99.99% including about 5 ppm of potassium and preferably at least 99.999% including about 0.1 ppm of potassium. As a result, the metallic impurities that vaporize from the tip of the anode and adhere to the inner wall of the tube are very scant, and even after long hours of being lit at high temperatures, there is almost no darkening or loss of transparency in the tube, and deterioration of the light output is suppressed.
There is less vaporization of metallic impurities if the cathode is also formed of high-purity tungsten. However, if the purity of the cathode is high and there are very few impurities, the work function of the tip of the cathode is increased, causing an increase of temperature of the cathode tip, which has a small heat capacity, and wastage actually increases. Therefore, in accordance with the present invention, the cathode is composed of tungsten doped with preferably potassium. However, potassium is an alkali metal, and thus, vaporizes yielding positive ions with a valance of 1. The positive ions are attracted to the cathode, which is electrically negative, and create a layer on the surface of the cathode. In other words, when the lamp is lighted stably a certain amount of the potassium with which the cathode is doped vaporizes but does not disperse to the inner surface of the tube; in effect it functions as an emitter. Accordingly, it is possible to suppress wastage of the cathode tip by doping with potassium so the purity of the cathode is not too high.
In addition, the temperature of the cathode coil that is composed of tungsten and wrapped around the cathode from which the arc begins to jump when the lamp is ignited rises quickly. Although the mass of the cathode coil is small, dispersion of the metallic impurities included in the cathode coil would have a deleterious effect. Therefore, it is desirable that the cathode coil, like the anode, be made of tungsten with a purity of at least 99.99%.